1. Field of the Invention
The present invention relates to a sewing machine having an automatic threader that transfers a needle thread drawn from a thread supply by a thread transferring mechanism to pass the needle thread through a plurality of thread passing sections.
2. Description of the Related Art
In a conventional sewing machine, a needle thread drawn from a thread spool is passed in a predetermined sequence and path and finally transferred to a sewing needle attached to a needle bar. The needle thread is then passed through a thread eye of the sewing needle and is ready to sew. In case the sewing machine is provided with a threading mechanism, an automatic threading is performed so that the needle thread is passed through the thread eye.
Various types of sewing machines have been put into practice wherein the needle thread drawn from the thread spool is manually passed through a plurality of predetermined thread passing sections on starting the sewing process. As opposed to this, sewing machines, in which the needle thread drawn from the thread spool is passed through a predetermined thread preparation path in advance and then automatically transferred by the thread transferring mechanism to be passed through a plurality of thread passing sections sequentially.
In case of passing the thread through the thread tension regulator among the plurality of thread passing sections, the thread tension regulator needs to be opened, that is, a pair of thread tension discs need to be spaced apart from each other. For example, in JP-A-2004-24729(entitled SEWING MACHINE WITH THREAD TENSION CONTROL FUNCTION AND THREAD TENSION CONTROL PROGRAM THEREFOR), a thread cassette accommodating a thread spool is detachably attached to a cassette attachment portion; and a thread tension control mechanism and pulse motor etc. is provided to drive the thread tension regulator to open/close. On detaching the thread cassette, the thread tension regulator is switched to an open state by the pulse motor. On the other hand, in case the thread cassette is attached when the thread tension regulator is opened, the pulse motor is driven after the elapse of a predetermined time period and the thread tension regulator is automatically switched to close.
In the sewing device described in the above-noted publication, on attaching the thread cassette, the thread tension regulator is kept open and automatically switched to close after the thread cassette is attached. That is, the opening and closing of the thread tension regulator associated with the attachment of the thread cassette is performed. On attaching the thread cassette, in case the thread is passed though a plurality of thread passing sections, further in case, passing the needle thread through the thread eye of the needle, the sewing device does not change the thread tension applied by the thread tension regulator accordingly.
Therefore, in case of passing the needle thread through the predetermined thread passing sections or threading the sewing thread, if the tension of the needle thread is loosely set, passing the needle thread through the plurality of thread passing sections and the needle threading are likely to fail. Such being the case, on thread passing or needle threading, the provision of such a high thread tension that does not fail the thread passing or needle threading is a possible solution.
However, when a high thread tension is applied, on passing the needle thread through the needle bar thread guide attached on the sewing needle by the threading mechanism, or threading the sewing needle by the needle threading mechanism, the thread supply from the thread spool is stopped and required thread amount cannot be supplied. This not only interrupts the thread passing operations of the threading mechanism but also the needle threading operations of the threading mechanism, in which case also faces problems such as the lack of reliability in performing the thread hook by the thread hooking mechanism and needle threading by the threading mechanism.